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Internal stress in aluminium oxide, titanium carbide and copper films obtained by planar magnetron sputtering
Thin Solid Films, 78 (1981)41-47 METALLURGICALAND PROTECTIVECOATINGS
41
INTERNAL STRESS IN ALUMINIUM OXIDE, TITANIUM CARBIDE AND COPPER FILMS OBTAINED BY PLANAR MAGNETRON SPUTTERING KAIZOKUXVAHARA,TSUNETAKASUMOMOGIAND MITSUNORIKONDO Faculty of Engineering, Hiroshima University, Hiroshima (Japan)
(ReceivedJune23, 1980;acceptedOctober 15, 1980)
Internal stresses in sputter-deposited aluminium oxide, titanium carbide and copper films were measured. Aluminium oxide and titanium carbide films developed compressive stresses whereas copper films exhibited tensile stresses. Their magnitudes and signs were found to be in good agreement with those of the thermal stresses estimated from the constants and deposition temperatures of the materials. It is suggested that an atomic peening is one of the verifiable mechanisms whereby the compressive component of the stresses is induced in these films,
1. INTRODUCTION Early investigations 1-5 on the internal stresses in vapour-deposited films have dealt with thermally evaporated metals, which are predominantly in a state of tension. Recent investigations6-1 o on the stresses of sputtered metals have revealed a more complex picture, however: states of both tension and compression and sometimes transitions between the two are observed. Although the internal stress is assumed to originate partly from an unequal contraction of the substrates and films upon cooling after deposition and partly from incomplete structural ordering during film growth 11, a detailed explanation has yet to be fully developed. High internal stress causes excessive bending of the underlying substrates, a change in the mechanical properties of the substrate materials 1° and in extreme cases the peeling off of films from the substrates or the failure of the substrates. Films thinner than the critical thickness for peeling can still be peeled off if the substrates are subjected to a finite amount of deformation. This peeling off thickness can decrease with increasing deformation. Such a tendency for film peeling is not desirable in the application of hard materials such as aluminium oxide for wearresistant coatings since rather thick coatings are required for this purpose. The stresses in hard coatings deposited by conventional r.f. sputtering do not seem to have been fully studied. The present investigation was thus undertaken to examine the stresses developed in such hard coatings and to compare them with the stresses in soft coatings (copper) deposited by r.f. sputtering as well as by vacuum evaporation. 0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedin The Netherlands
42
K. KUWAHARA, T. SUMOMOGI, M. KONDO
2. EXPERIMENTAL DETAILS Substrates used for the stress measurements were cover glass wafers 18 m m in diameter and 0.15 m m thick which were selected for their flatness. The substrates were ultrasonically degreased in acetone and mounted on a substrate electrode of a planar magnetron system equipped with a target electrode 100 m m in diameter. After the chamber was evacuated to a pressure of 2 x 10- 3 Pa, the working gas, argon, was introduced and sputtering was commenced when the argon gas pressure was stabilized to a value between 4 and 6 Pa. The pressure was maintained constant during the sputtering and the r.f. power input was kept at 200 W (2.5 W cm-2). Aluminium oxide, titanium carbide and copper films were deposited from a target of each material onto the glass substrates. Although the deposition rate was not controlled, it was determined from the input power and the pressure. The deposition rates were about 0.3 ~tm h - 1 at 4 Pa for aluminium oxide, about 0.5 ~tm h - 1 at 4.5 Pa for titanium carbide, and about 5 ~tm h - 1 at 6.5 Pa for copper. These pressure values were chosen so as to obtain the highest deposition rate for each film. The substrates were not heated but their temperatures were raised to 200 °C by radiation from the target and by b o m b a r d m e n t s by atoms, ions and electrons. The induced stresses (i.e. the average stresses a, which are the internal stresses of the films averaged across the thickness, and the total stresses S, which are defined as the average stresses multiplied by the thickness) were determined using the revised Stoney equation12 Ed 2
o - - 6(1 -
v)rt
(1)
and S = at
(2)
where E, v and d are the Young modulus, the Poisson ratio and the thickness respectively of the substrate, t is the film thickness and r is the radius of the curvature of the coated wafer. In the present work the substrate glass used had values of E of 7.1 × 10 l° Pa, v = 0.23 and d = 0.15 mm. The curvature was measured with a light section profilometer; the wafer was placed under a microscope and was illuminated by light through a narrow slit at an angle of incidence so that the profile of the wafer was observed as a bright line or a curve. The accuracy of the measured total stress was of the order of 10 N m - 1. For the purpose of comparing their stresses with those of the aforementioned coatings, additional specimens were prepared. These include films of aluminium oxide and titanium carbide sputter deposited onto high speed steel substrates and films of copper vacuum deposited onto glass substrates. 3. EXPERIMENTAL RESULTS The thickness dependences of the average stresses tr and total stresses S of aluminium oxide films are shown in Fig. 1. As can be seen in the figure, the average compressive stress decreases gradually with increasing thickness, whereas the total stress increases more rapidly. At a film thickness of greater than 10 ~tm the glass substrate is fractured into pieces even during deposition. The surface tension or
43
I N T E R N A L STRESS IN FILMS OBTAINED BY M A G N E T R O N S P U T T E R I N G
compression necessary to break the substrate was estimated from the yield or fracture strength of glass and was found to coincide with the total stress ( ~ 103 N m - 1 ) at the critical thickness. The thickness dependences of tr and S for titanium carbide films is shown in Fig. 2. In this case both S and tr increase with increasing thickness. Above a thickness of 3 ixm the glass substrate is fractured. Again the total stress at the critical thickness was about 103 N m - t.
ld
104
10g
10' /
~.A¢-~ ~
z
? ,o' z
10I
~1o'
| IIJ
~/o.1
I I I Illll
I
1 10 Film Thickness ( p r o )
10
10~1
I
I liilllll
I
10
70
Film Thickness (IJm)
Fig. 1. Total stress or force per unit width S and average stress tr in sputtered aluminium oxide films as functions of thickness. For film thicknesses of above 10 Ixm the glass substrates were fractured. Fig. 2. Total stress S and average stress tr in sputtered titanium carbide films as functions of thickness. For thicknesses of above 3 ttm the glass substrates were fractured.
In the case of copper films, as shown in Fig. 3, a tensile stress develops rather than the compressive stress observed in the former two coatings. The behaviour of the absolute ~ and S values is not much different from that for aluminium oxide films, however. Fracture of the glass substrate takes place at a thickness of about 16 ~tm and the total stress at that thickness is almost the same as the value obtained in the former two cases. F o r the sake of comparison, the thickness dependence of the internal stresses in the vacuum-deposited copper films is shown in Fig. 4. It can be seen that the stresses are in a state of tension and their thickness dependence is similar to that for sputtered copper films. The rate of increase of the total stress with thickness is higher and consequently the critical thickness, above which substrates are fractured, is as low as 5 ~tm (16 ~tm for sputter-deposited copper films). Another experiment on the thickness dependence of aluminium oxide films sputter deposited onto high speed steel substrates revealed that the dependence is not very different from that of aluminium oxide films deposited on glass substrates (see Fig. 1). 4. DISCUSSION An inspection of Figs. 1-4 suggests that the thermal stress component makes an important contribution to the measured stress--at least to its state (compressive or
44
K . K U W A H A R A , T. S U M O M O G I , M. K O N D O
Io'
IO'
io"
I0 s
g-
E
Z
Z
E
10' z
~10'
10~ v
L.
g
i.
i,o,
o 1o,
~>
<
1~0.1
I
I IIIIlil
1
10
1
I0
107.1-0
I
Film Thickness (pro)
I I I
lira
1 Film Thickness (pro)
I 10
I0
Fig. 3. Total stress S and average stress tr in sputtered copper films as functions of thickness. For film thicknesses of above 16 Ima the glass substrates were fractured. Fig. 4. Total stress S and average stress o in evaporated copper films as functions of thickness. For thicknesses of above 5 pm the glass substrates were fractured.
tensile). Rough estimates of the state and magnitude of the thermal stress ath and the stress tr. . . . measured in thick films are shown in Table I together with the values of thermal expansion coefficients ~ and Young's moduli E for each material from refs. 13 and 14. The deposition temperature is assumed to be 200 °C. Values of~ and E are not well known, especially for aluminium oxide and titanium carbide, and furthermore there is a possibility that they could be slightly different from the corresponding bulk values. Accordingly the approximate values are shown in the table. Nevertheless, except for titanium carbide films, the agreement between the estimated and measured stresses is fairly good. It has been found from the result of X-ray photoelectron spectroscopy and electron probe microanalysis investigations that the carbon content in the titanium carbide films is less than that of the stoichiometric TiC compound. This difference in carbon content would affect the values of ~ and E; this might account partly for the aforementioned discrepancy. TABLE I V A L U E S O F T H E R M A L E X P A N S I O N C O E F F I C I E N T S 0£ A N D Y O U N G ' S M O D U L I E F O R F I L M M A T E R I A L S ( R O U G H VALUES), ESTIMATED THERMALLY INDUCED INTERNAL STRESSES ath , AND MEASURED INTERNAL STRESSES
ameas F O R T H I C K F I L M S a
Materials
Aluminium oxide Titanium carbide Copper Glass
~ x 10 6
E x 10-1 x
ath x 1 0 - a
amc,, x I 0 - s
( K - 1)
(Pa)
(Pa)
(Pa)
7 6 17
4 4-5 1
- 1.4 - (2.2-2.7) + 1.4
+ +
9
a T h e values o f ~ and E were obtained mainly from refs. 13 and 14.
1.5 8 0.8 (sputtered), 1.5 (evaporated)
45
INTERNAL STRESS IN FILMS OBTAINED BY MAGNETRON SPUTTERING
It has recently been postulated 6"7,15 that peening by atoms and ions of the working gas during sputtering is what causes the compressive stresses in the sputterdeposited films to develop. Incorporation of gas atoms into the films may also cause a lattice expansion resulting in the development of compressive stresses s' 16,17. This peening and incorporation of atoms would partly account for the smaller tensile stresses in the sputtered copper films than in the evaporated copper films. In the present investigation the stress dependence on the deposition rate was also examined. In this experiment the deposition rate was varied by changing the working gas pressure under the application of a constant r.f. input power (200 W). The results at the thickness of about 1 pm are shown in Figs. 5-7 for aluminium oxide, titanium carbide and copper respectively. Although the results include the pressure dependence implicitly, there is an overall trend which indicates that the higher rate brings about higher tensile stresses or lower compressive stresses. This trend may also be caused by a peening, because the number of particle peening events increases with increasing deposition time. The incorporation of gas atoms into films may make some contribution to the dependence of the stress on the deposition rate.
zl0, == P ~l~lOs
0~101
10:
O
a
I 0.1
~
Deposition Rote
I 0.2 (pmlh)
,
I 0.2
,
I 0.4
Deposition Rote
, (pmlh)
Fig. 5. Average stress of sputtered aluminium oxide films on glass substrates as a function of deposition rate. Fig. 6. Average stress of sputtered titanium carbide films on glass substrates as a function of deposition rate.
As mentioned in the preceding section, the stresses in the films deposited on high speed steel substrates show a dependence similar to that of the stresses in films on glasses. This may be due to the similar values of thermal expansion coefficients for both substrates. In our experiments the internal stresses were studied only with the films deposited on thin substrates. In practical applications for wear-resistant coatings, however, the substrates are usually so thick that very little substrate bending or bulging can occur in association with the deposition of films and thus little relaxation of the stresses is expected. This factor should be taken into account if the films are deposited on thick substrates.
46
K. KUWAHARA, T. SUMOMOGI, M. KONDO
~101
Yt
I I 005 0.1 Deposition Rate ( p m l m i n )
Fig. 7. Average stress of sputtered copper films on glass substrates as a function of deposition rate. 5. CONCLUSIONS
Thick aluminium oxide, titanium carbide and copper films were deposited on glass wafers using a planar magnetron sputtering apparatus and their internal stresses were measured from the curvature of the wafers. The following results were obtained. (1) Internal stresses in aluminium oxide films were compressive; the average stresses decreased with increasing film thickness while the total stresses increased. (2) The average stresses and the total stresses in titanium carbide films, being compressive, increased with increasing film thickness. (3) In the case of copper films, the behaviour was similar to that of aluminium oxide films, except that the stresses were tensile. (4) In all cases the substrate glasses were fractured during deposition, at film thicknesses of 10 ~tm for aluminium oxide, 3 ~tm for titanium carbide and 16 Ixm for copper respectively. The total stresses in these thicknesses were the same and corresponded to the fracture strength of the substrate glass. (5) The observed stresses were attributed primarily to the thermal stresses. A peening by atoms, ions and electrons during film growth was presumed to increase the compressive stress component in these films. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
G.P. Weiss and D. O. Smith, J. Appl. Phys., 33 (1962) 1166. R. Glang, R. A. Holmwood and R. L. Rosenfeld, Rev. Sci. Instrum., 36 (1965) 7. K. Kinosita, K. Maki, K. Nakamizo and K. Takeuchi, Jpn. J. AppL Phys., 6 (1967) 42. E. Klokholm, Rev. Sci. Instrurn., 40 (1969) 1054. K. Maki, Y. Nakajima and K. Kinosita, J. Vac. Sci. Technol., 6 (1969) 622. D . W . HoffmanandJ. A. Thornton, ThinSolidFilms, 45(1977)387. J.A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., 14 (1977) 164. C.T. Wu, Thin Solid Films, 64 (1979) 103. S.M. K a n e a n d K . Y. Ahn, J. Vac. Sci. Technol.,16(1979) 171. P.V. Plunkett, R. M. Johnson and C. D. Wiseman, Thin Solid Films, 64 (1979) 121. R. W. Hoffman, Phys. Thin Films, 3 (1966) 211. J.D. Finegan, AEC Tech. Rep. 15, Case Institute of Technology, Cleveland, Ohio, 1961. American Institute o f Physics Handbook, McGraw-Hill, New York, 3rd edn., 1972.
INTERNAL STRESS IN FILMS OBTAINED BY MAGNETRON SPUTTERING
14 Handb••k •f Tab•es f•r App•ied Engineering Science• Chemica• Rubber C•mpany• C•eve•and• 2nd edn., 1973. 15 J.A. Thornton, J. TabockandD. W. Hoffman, ThinSolidFilms, 64(1979) lll. 16 H.F. Winters and E. Kay, J. Appl. Phys., 38 (1967) 3928. 17 W.W. Lee and D. Oblas, J. Vac. Sci. Technol., 7 (1970) 129.
47 •hi••
41
INTERNAL STRESS IN ALUMINIUM OXIDE, TITANIUM CARBIDE AND COPPER FILMS OBTAINED BY PLANAR MAGNETRON SPUTTERING KAIZOKUXVAHARA,TSUNETAKASUMOMOGIAND MITSUNORIKONDO Faculty of Engineering, Hiroshima University, Hiroshima (Japan)
(ReceivedJune23, 1980;acceptedOctober 15, 1980)
Internal stresses in sputter-deposited aluminium oxide, titanium carbide and copper films were measured. Aluminium oxide and titanium carbide films developed compressive stresses whereas copper films exhibited tensile stresses. Their magnitudes and signs were found to be in good agreement with those of the thermal stresses estimated from the constants and deposition temperatures of the materials. It is suggested that an atomic peening is one of the verifiable mechanisms whereby the compressive component of the stresses is induced in these films,
1. INTRODUCTION Early investigations 1-5 on the internal stresses in vapour-deposited films have dealt with thermally evaporated metals, which are predominantly in a state of tension. Recent investigations6-1 o on the stresses of sputtered metals have revealed a more complex picture, however: states of both tension and compression and sometimes transitions between the two are observed. Although the internal stress is assumed to originate partly from an unequal contraction of the substrates and films upon cooling after deposition and partly from incomplete structural ordering during film growth 11, a detailed explanation has yet to be fully developed. High internal stress causes excessive bending of the underlying substrates, a change in the mechanical properties of the substrate materials 1° and in extreme cases the peeling off of films from the substrates or the failure of the substrates. Films thinner than the critical thickness for peeling can still be peeled off if the substrates are subjected to a finite amount of deformation. This peeling off thickness can decrease with increasing deformation. Such a tendency for film peeling is not desirable in the application of hard materials such as aluminium oxide for wearresistant coatings since rather thick coatings are required for this purpose. The stresses in hard coatings deposited by conventional r.f. sputtering do not seem to have been fully studied. The present investigation was thus undertaken to examine the stresses developed in such hard coatings and to compare them with the stresses in soft coatings (copper) deposited by r.f. sputtering as well as by vacuum evaporation. 0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedin The Netherlands
42
K. KUWAHARA, T. SUMOMOGI, M. KONDO
2. EXPERIMENTAL DETAILS Substrates used for the stress measurements were cover glass wafers 18 m m in diameter and 0.15 m m thick which were selected for their flatness. The substrates were ultrasonically degreased in acetone and mounted on a substrate electrode of a planar magnetron system equipped with a target electrode 100 m m in diameter. After the chamber was evacuated to a pressure of 2 x 10- 3 Pa, the working gas, argon, was introduced and sputtering was commenced when the argon gas pressure was stabilized to a value between 4 and 6 Pa. The pressure was maintained constant during the sputtering and the r.f. power input was kept at 200 W (2.5 W cm-2). Aluminium oxide, titanium carbide and copper films were deposited from a target of each material onto the glass substrates. Although the deposition rate was not controlled, it was determined from the input power and the pressure. The deposition rates were about 0.3 ~tm h - 1 at 4 Pa for aluminium oxide, about 0.5 ~tm h - 1 at 4.5 Pa for titanium carbide, and about 5 ~tm h - 1 at 6.5 Pa for copper. These pressure values were chosen so as to obtain the highest deposition rate for each film. The substrates were not heated but their temperatures were raised to 200 °C by radiation from the target and by b o m b a r d m e n t s by atoms, ions and electrons. The induced stresses (i.e. the average stresses a, which are the internal stresses of the films averaged across the thickness, and the total stresses S, which are defined as the average stresses multiplied by the thickness) were determined using the revised Stoney equation12 Ed 2
o - - 6(1 -
v)rt
(1)
and S = at
(2)
where E, v and d are the Young modulus, the Poisson ratio and the thickness respectively of the substrate, t is the film thickness and r is the radius of the curvature of the coated wafer. In the present work the substrate glass used had values of E of 7.1 × 10 l° Pa, v = 0.23 and d = 0.15 mm. The curvature was measured with a light section profilometer; the wafer was placed under a microscope and was illuminated by light through a narrow slit at an angle of incidence so that the profile of the wafer was observed as a bright line or a curve. The accuracy of the measured total stress was of the order of 10 N m - 1. For the purpose of comparing their stresses with those of the aforementioned coatings, additional specimens were prepared. These include films of aluminium oxide and titanium carbide sputter deposited onto high speed steel substrates and films of copper vacuum deposited onto glass substrates. 3. EXPERIMENTAL RESULTS The thickness dependences of the average stresses tr and total stresses S of aluminium oxide films are shown in Fig. 1. As can be seen in the figure, the average compressive stress decreases gradually with increasing thickness, whereas the total stress increases more rapidly. At a film thickness of greater than 10 ~tm the glass substrate is fractured into pieces even during deposition. The surface tension or
43
I N T E R N A L STRESS IN FILMS OBTAINED BY M A G N E T R O N S P U T T E R I N G
compression necessary to break the substrate was estimated from the yield or fracture strength of glass and was found to coincide with the total stress ( ~ 103 N m - 1 ) at the critical thickness. The thickness dependences of tr and S for titanium carbide films is shown in Fig. 2. In this case both S and tr increase with increasing thickness. Above a thickness of 3 ixm the glass substrate is fractured. Again the total stress at the critical thickness was about 103 N m - t.
ld
104
10g
10' /
~.A¢-~ ~
z
? ,o' z
10I
~1o'
| IIJ
~/o.1
I I I Illll
I
1 10 Film Thickness ( p r o )
10
10~1
I
I liilllll
I
10
70
Film Thickness (IJm)
Fig. 1. Total stress or force per unit width S and average stress tr in sputtered aluminium oxide films as functions of thickness. For film thicknesses of above 10 Ixm the glass substrates were fractured. Fig. 2. Total stress S and average stress tr in sputtered titanium carbide films as functions of thickness. For thicknesses of above 3 ttm the glass substrates were fractured.
In the case of copper films, as shown in Fig. 3, a tensile stress develops rather than the compressive stress observed in the former two coatings. The behaviour of the absolute ~ and S values is not much different from that for aluminium oxide films, however. Fracture of the glass substrate takes place at a thickness of about 16 ~tm and the total stress at that thickness is almost the same as the value obtained in the former two cases. F o r the sake of comparison, the thickness dependence of the internal stresses in the vacuum-deposited copper films is shown in Fig. 4. It can be seen that the stresses are in a state of tension and their thickness dependence is similar to that for sputtered copper films. The rate of increase of the total stress with thickness is higher and consequently the critical thickness, above which substrates are fractured, is as low as 5 ~tm (16 ~tm for sputter-deposited copper films). Another experiment on the thickness dependence of aluminium oxide films sputter deposited onto high speed steel substrates revealed that the dependence is not very different from that of aluminium oxide films deposited on glass substrates (see Fig. 1). 4. DISCUSSION An inspection of Figs. 1-4 suggests that the thermal stress component makes an important contribution to the measured stress--at least to its state (compressive or
44
K . K U W A H A R A , T. S U M O M O G I , M. K O N D O
Io'
IO'
io"
I0 s
g-
E
Z
Z
E
10' z
~10'
10~ v
L.
g
i.
i,o,
o 1o,
~>
<
1~0.1
I
I IIIIlil
1
10
1
I0
107.1-0
I
Film Thickness (pro)
I I I
lira
1 Film Thickness (pro)
I 10
I0
Fig. 3. Total stress S and average stress tr in sputtered copper films as functions of thickness. For film thicknesses of above 16 Ima the glass substrates were fractured. Fig. 4. Total stress S and average stress o in evaporated copper films as functions of thickness. For thicknesses of above 5 pm the glass substrates were fractured.
tensile). Rough estimates of the state and magnitude of the thermal stress ath and the stress tr. . . . measured in thick films are shown in Table I together with the values of thermal expansion coefficients ~ and Young's moduli E for each material from refs. 13 and 14. The deposition temperature is assumed to be 200 °C. Values of~ and E are not well known, especially for aluminium oxide and titanium carbide, and furthermore there is a possibility that they could be slightly different from the corresponding bulk values. Accordingly the approximate values are shown in the table. Nevertheless, except for titanium carbide films, the agreement between the estimated and measured stresses is fairly good. It has been found from the result of X-ray photoelectron spectroscopy and electron probe microanalysis investigations that the carbon content in the titanium carbide films is less than that of the stoichiometric TiC compound. This difference in carbon content would affect the values of ~ and E; this might account partly for the aforementioned discrepancy. TABLE I V A L U E S O F T H E R M A L E X P A N S I O N C O E F F I C I E N T S 0£ A N D Y O U N G ' S M O D U L I E F O R F I L M M A T E R I A L S ( R O U G H VALUES), ESTIMATED THERMALLY INDUCED INTERNAL STRESSES ath , AND MEASURED INTERNAL STRESSES
ameas F O R T H I C K F I L M S a
Materials
Aluminium oxide Titanium carbide Copper Glass
~ x 10 6
E x 10-1 x
ath x 1 0 - a
amc,, x I 0 - s
( K - 1)
(Pa)
(Pa)
(Pa)
7 6 17
4 4-5 1
- 1.4 - (2.2-2.7) + 1.4
+ +
9
a T h e values o f ~ and E were obtained mainly from refs. 13 and 14.
1.5 8 0.8 (sputtered), 1.5 (evaporated)
45
INTERNAL STRESS IN FILMS OBTAINED BY MAGNETRON SPUTTERING
It has recently been postulated 6"7,15 that peening by atoms and ions of the working gas during sputtering is what causes the compressive stresses in the sputterdeposited films to develop. Incorporation of gas atoms into the films may also cause a lattice expansion resulting in the development of compressive stresses s' 16,17. This peening and incorporation of atoms would partly account for the smaller tensile stresses in the sputtered copper films than in the evaporated copper films. In the present investigation the stress dependence on the deposition rate was also examined. In this experiment the deposition rate was varied by changing the working gas pressure under the application of a constant r.f. input power (200 W). The results at the thickness of about 1 pm are shown in Figs. 5-7 for aluminium oxide, titanium carbide and copper respectively. Although the results include the pressure dependence implicitly, there is an overall trend which indicates that the higher rate brings about higher tensile stresses or lower compressive stresses. This trend may also be caused by a peening, because the number of particle peening events increases with increasing deposition time. The incorporation of gas atoms into films may make some contribution to the dependence of the stress on the deposition rate.
zl0, == P ~l~lOs
0~101
10:
O
a
I 0.1
~
Deposition Rote
I 0.2 (pmlh)
,
I 0.2
,
I 0.4
Deposition Rote
, (pmlh)
Fig. 5. Average stress of sputtered aluminium oxide films on glass substrates as a function of deposition rate. Fig. 6. Average stress of sputtered titanium carbide films on glass substrates as a function of deposition rate.
As mentioned in the preceding section, the stresses in the films deposited on high speed steel substrates show a dependence similar to that of the stresses in films on glasses. This may be due to the similar values of thermal expansion coefficients for both substrates. In our experiments the internal stresses were studied only with the films deposited on thin substrates. In practical applications for wear-resistant coatings, however, the substrates are usually so thick that very little substrate bending or bulging can occur in association with the deposition of films and thus little relaxation of the stresses is expected. This factor should be taken into account if the films are deposited on thick substrates.
46
K. KUWAHARA, T. SUMOMOGI, M. KONDO
~101
Yt
I I 005 0.1 Deposition Rate ( p m l m i n )
Fig. 7. Average stress of sputtered copper films on glass substrates as a function of deposition rate. 5. CONCLUSIONS
Thick aluminium oxide, titanium carbide and copper films were deposited on glass wafers using a planar magnetron sputtering apparatus and their internal stresses were measured from the curvature of the wafers. The following results were obtained. (1) Internal stresses in aluminium oxide films were compressive; the average stresses decreased with increasing film thickness while the total stresses increased. (2) The average stresses and the total stresses in titanium carbide films, being compressive, increased with increasing film thickness. (3) In the case of copper films, the behaviour was similar to that of aluminium oxide films, except that the stresses were tensile. (4) In all cases the substrate glasses were fractured during deposition, at film thicknesses of 10 ~tm for aluminium oxide, 3 ~tm for titanium carbide and 16 Ixm for copper respectively. The total stresses in these thicknesses were the same and corresponded to the fracture strength of the substrate glass. (5) The observed stresses were attributed primarily to the thermal stresses. A peening by atoms, ions and electrons during film growth was presumed to increase the compressive stress component in these films. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
G.P. Weiss and D. O. Smith, J. Appl. Phys., 33 (1962) 1166. R. Glang, R. A. Holmwood and R. L. Rosenfeld, Rev. Sci. Instrum., 36 (1965) 7. K. Kinosita, K. Maki, K. Nakamizo and K. Takeuchi, Jpn. J. AppL Phys., 6 (1967) 42. E. Klokholm, Rev. Sci. Instrurn., 40 (1969) 1054. K. Maki, Y. Nakajima and K. Kinosita, J. Vac. Sci. Technol., 6 (1969) 622. D . W . HoffmanandJ. A. Thornton, ThinSolidFilms, 45(1977)387. J.A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., 14 (1977) 164. C.T. Wu, Thin Solid Films, 64 (1979) 103. S.M. K a n e a n d K . Y. Ahn, J. Vac. Sci. Technol.,16(1979) 171. P.V. Plunkett, R. M. Johnson and C. D. Wiseman, Thin Solid Films, 64 (1979) 121. R. W. Hoffman, Phys. Thin Films, 3 (1966) 211. J.D. Finegan, AEC Tech. Rep. 15, Case Institute of Technology, Cleveland, Ohio, 1961. American Institute o f Physics Handbook, McGraw-Hill, New York, 3rd edn., 1972.
INTERNAL STRESS IN FILMS OBTAINED BY MAGNETRON SPUTTERING
14 Handb••k •f Tab•es f•r App•ied Engineering Science• Chemica• Rubber C•mpany• C•eve•and• 2nd edn., 1973. 15 J.A. Thornton, J. TabockandD. W. Hoffman, ThinSolidFilms, 64(1979) lll. 16 H.F. Winters and E. Kay, J. Appl. Phys., 38 (1967) 3928. 17 W.W. Lee and D. Oblas, J. Vac. Sci. Technol., 7 (1970) 129.
47 •hi••